1. Field of the Invention
The present invention relates to an apparatus for driving a piston by fluid pressure such as pneumatic pressure or hydraulic pressure.
2. Description of Prior Art
Conventionally, as an apparatus for driving a piston by fluid pressure which is a subject for the present invention there has been known the one disclosed in U.S. Pat. No. 5,050,482. This is the apparatus previously proposed by the assignee of the present invention and its basic constitution is as follows.
As illustrated in a system view of FIG. 5, a piston 8 is vertically movably inserted into a cylinder 7. A driving chamber 9 is arranged between an upper wall 7a of the cylinder 7 and the piston 8. Pressure fluid is supplied to and discharged from the driving chamber 9 by a supply-discharge valve 13. The supply-discharge valve 13 is adapted to be switched between a supply position X of the pressure fluid and a discharge position Y thereof by a pilot valve 18. The symbol 14 designates a pressure supply port of the pressure fluid and the symbol 15 does a pressure discharge port thereof.
The basic constitution will be explained in detail with reference to FIG. 6 illustrating the apparatus disclosed in the above-mentioned prior art publication hereinafter.
The supply-discharge valve 13 comprises a cylindrical supply-discharge valve casing 29 disposed above the cylinder 7 and a supply-discharge valve member 30 vertically movably inserted into the supply-discharge valve casing 29. A supply actuation chamber 33 communicated with the pressure supply port 14 is arranged below the supply-discharge valve member 30 and a discharge actuation chamber 35 to be selectively communicated with the pressure supply port 14 and a pressure relief port 55 is arranged above the supply-discharge valve member 30.
The pilot valve 18 comprises a sleeve 44 inserted into a bore 30d of the supply-discharge valve member 30, a spool valve member 46 vertically movably inserted into the sleeve 44, an annular sealing member 48 arranged between the pressure supply port 14 and the discharge actuation chamber 35 and a pressure relief valve member 57 arranged between the discharge actuation chamber 35 and the pressure relief port 55. The annular sealing member 48 is fitted between an outer peripheral surface of the spool valve member 46 and a lower portion of the bore 30d so as to be brought into contact with a lower portion of the sleeve 44 from below. Further, the lower portion of the spool valve member 46 is fixedly secured to the piston 8.
As shown in FIGS. 5 and 6, the apparatus 2 for driving the piston by the fluid pressure having the basic constitution operates as follows.
When a pressure fluid supply valve 16 is opened, a pressure fluid such as a pressure air or a pressure oil is supplied from a fluid pressure source 17 to operate the driving apparatus 2. When the valve 16 is closed, the pressure fluid supply is stopped and then the operation of the driving apparatus 2 is stopped.
As shown in the left half view of FIG. 6, while the operation is stopped, the piston 8 and the spool valve member 46 are pushed back to the top dead center by a return spring 11, so that the supply-discharge valve member 30 is pushed up to the supply position X.
While the operation is continued, a descending drive stroke illustrated in the left half view thereof and an ascending return stroke illustrated in the right half view thereof are repeated.
During the descending drive stroke, since the pressure relief valve member 57 is opened and the pressure fluid within the discharge actuation chamber 35 is released from the pressure relief port 55 to the pressure discharge port 15, the supply-discharge valve member 30 is pushed up by the fluid pressure of the supply actuation chamber 33 to the supply position X on the upper side and the pressure fluid always supplied to the supply actuation chamber 33 is forced into a driving chamber 9 from a working chamber 32 to descend the piston 8.
During the ascending return stroke, when the piston 8 is near the bottom dead center, as shown in the right half view thereof, the annular sealing member 48 is opened and the pressure fluid always supplied from the pressure supply port 14 is introduced into the discharge actuation chamber 35 through the sleeve 44, so that the supply-discharge valve member 30 is pushed down by the fluid pressure to the discharge position Y on the lower side and the pressure fluid within the driving chamber 9 is released from the working chamber 32 to the pressure discharge port 15 through a discharge chamber 34 to ascend and return the piston 8 by the return spring 11. Thus, when the piston 8 reaches the top dead center, as shown in the left half view thereof, the pressure relief valve member 57 is opened, switching over to the descending drive stroke.
In the basic constitution, conventionally the pilot valve 18 is further constituted as follows.
As shown in FIG. 6, a cylinder bore 91 to be communicated with the discharge actuation chamber 35 is formed vertically in an upper portion of the supply-discharge valve casing 29, a piston 92 formed in an upper portion of the sleeve 44 is airtightly inserted into the cylinder bore 91 through an O-ring 93, a pressure receiving chamber 94 is formed below the piston 92, and a return spring 95 for urging the sleeve 44 downward is provided.
As noted above, the prior art has such an advantage that the driving apparatus 2 can be prevented from stopping at an extremely low speed.
That is, as shown in FIG. 5, while a hydraulic pump 3 of the plunger type is driven by the driving apparatus 2 to continue the pressure fluid supply even after completion of an extension of a hydraulic cylinder 86, when an extremely small amount of pressure oil leaks from a hydraulic actuation chamber 87, a switching valve 88 or the like or an extremely small amount of pressure oil enters a seal clearance of a sealing member, the piston 8 drives a plunger 22 of the hydraulic pump 3 at an extremely slow speed to supplement that extremely small amount of pressure oil.
When the piston 8 is driven at the extremely slow speed in this way to access the bottom dead center and the spool valve member 46 passes by the annular sealing member 48 at the extremely slow speed to separate therefrom a small distance, the pressure fluid within the pressure supply port 14 flows into the discharge actuation chamber 35 to slowly push down the supply-discharge valve member 30 by a force corresponding to a pressure imposed onto a discharge pressure receiving surface 30c. On a midway of that slow pushing down, since the working chamber 32 is communicated with both the supply actuation chamber 33 and the discharge chamber 34, the pressure fluid within the driving chamber 9 is released from the working chamber 32 to the pressure discharge port 15.
Therefore, provided that a descending speed of the supply-discharge valve member 30 is extremely slow, the piston 8 is pushed up by a resilient force of the return spring 11 before completion of its descending stroke and the spool valve member 46 closes the sealing member 48 again on a midway of opening. Thereupon, a low pressure fluid is enclosed within the discharge actuation chamber 35 as well as the pressure fluid within the supply actuation chamber 33 is discharged from the working chamber 32 to the discharge chamber 34 along a shortcircuit. As a result, the supply-discharge valve member 30 stops on a midway of descending due to a balance between a pushing-down force applied from the discharge actuation chamber 35 and a pushing-up force applied from the supply actuation chamber 33, so that it becomes impossible to drive the piston 8 downward and the driving apparatus 2 is stopped.
But, according to the above-mentioned prior art, when the spool valve member 46 is descended at the extremely slow speed and its outer peripheral surface separates from an inner peripheral surface of the annular sealing member 48 at the extremely slow speed, the pressure fluid within the pressure supply port 14 is introduced into the sleeve 44 through a valve opening clearance between the spool valve member 46 and the sealing member 48 to gradually increase a pressure within the pressure receiving chamber 94 at the extremely slow speed. Thereupon, when the pressure within the pressure receiving chamber 94 reaches a predetermined pressure, as indicated by a solid line in the right half view, since the sleeve 44 is ascended by that internal pressure against two springs 58, 95 so that also the sealing member 48 is pushed up accompanying therewith, the sealing member 48 is quickly separated from the spool valve member 46.
Thereupon, the pressure fluid within the pressure supply port 14 is introduced into the discharge actuation chamber 35 through the large valve opening clearance to quickly increase the pressure within the discharge actuation chamber 35, to strongly push down and quickly descend the supply-discharge valve member 30 by the increased pressure and to switch the supply-discharge valve member 30 to the discharge position Y in the right half view. Since the supply-discharge valve member 30 is strongly pushed down and quickly descended in that way, its midway stop during descending can be prevented. As a result, it is possible to prevent the driving apparatus 2 from falling into an abnormal stop.
In this way, the prior art has such an advantage that the driving apparatus 2 can be prevented from stopping even when being driven at the extremely slow speed. But, there still remains a problem to be improved as follows.
That is, since the annular sealing member 48 has its inner peripheral surface adapted to come into slidable contact with an outer peripheral surface of the spool valve member 46 and its outer peripheral surface adapted to come into slidable contact with the bore 30d, wearing-out is increased as a total operation time of the driving apparatus 2 becomes longer, so that the sealing performance degrades.
While the piston 8 is stopped at a midway height by an increase of pressure within a pump chamber 21 during the descending drive of the piston 8, when the pressure fluid within the pressure supply port 14 leaks into the sleeve 44 due to the degradation of the sealing performance of the sealing member 48, the leaked pressure fluid increases the pressure within the discharge actuation chamber 35 at the extremely slow speed, so that the supply-discharge valve member 30 is descended at the extremely slow speed by that increased pressure. Therefore, due to the same reason as that described above, the supply-discharge valve member 30 stops at a midway descend position, so that the driving apparatus 2 falls into the abnormal stop.